By replacing the cement with metakaolin in hydroxyethyl methyl cellulose modified mortar in 0%， 5%,10% and 15%（by mass）and controlling the fresh mortar consistency by regulating the water demand, the effects of metakaolin on properties of fresh and hardened mortars were investigated. The results show that the higher the replacement ratio of metakaolin, the higher the mass ratio of water to binder, the shorter the setting time of fresh mortar become while its good water retention and workability maintain. By replacing the cement with metakaolin, the compressive strength and impermeability of hardened mortar increase with an optimized pore diameter distribution. The effect of metakaolin on the flexural strength is little. The dry shrinkage value decrease with the increase of the replacement ratio of metakaolin from 5% to 15%. When the replacement ratio of metakaolin reaches 10%, the hardened mortar could obtain its best performance.